Microsoft has posted up a partial list of what's new, improved and fixed in the new Windows 10 Mobile preview build 10512. The list includes quite a few improvements but there doesn't appear to be any major new features in this build.

Here's the list, as posted by Microsoft's Windows Insider head Gabriel Aul:

General improvement in overall stability and performance.

You can now set the Lock screen and background image from within the Photos app.

We fixed an issue where apps installed on SD cards weren't working after a reboot.

We have improved Latvian and Chinese inputs.

We have made improvements to the Kid's Corner tile layout.

We have improved reliability of Data Sense notifications.

Modified Shape Writing algorithm to be less aggressive in suggesting contact names over common words.

Resolved an issue where Camera would not launch when the phone was locked.

We fixed an issue where notifications for incoming text messages weren't appearing.

We fixed an issue where the touch screen wouldn't be responsive after ending a phone call on some devices.

And we fixed an issue where folder tiles have obviously overlapping text.

The unresponsive screen after/during calls was pretty much my only serious point of frustration at this point. If that's truly been corected then bring on Talkman! I'll be eligible for na upgrade at the end of the month too mwahahhaa

Am I the only one who don't see W10 Mobile with a good eye?
With the multiples bugs build after build, I'm a little worry for the future of W10 Mobile. Of course it's only Beta now but we are @ 10512. It should be very stable, but...

The fix for touchscreen freeze after hanging up is VERY important. The consequences of this bug, or any other that inexplicably and suddenly disables the phone function, could be very dangerous; better not need to dial 911. And, beyond this, ya gotta wonder why it's so hard for MS to implement quality apps that are so common and conventional as email and photos. You would think they would have been able to knock those out, top of the line, before they moved on to pretty much anything else.

Give me performance over features anytime! let's be realistic here, would you honestly want all new features in Beta Testing? I'm sure most of you would say yes but where's the surprise in that. I'm sure the people at MS are listening and will WOW you when it's announcement time for Win 10 mobile and New phone line up. until then....be patient young beta tester..

The entire point of the insider program is to gather public feedback on new features and changes to the operating system, not to squash bugs. So let's be realistic here, while performance enhancement is good it is second fiddle to new features; enhanced real world bug reporting is only a happy side effect of the insider program.

Microsoft has also stated, if I remember correctly, that Windows 10 mobile is mostly feature complete, and while they will be adding things right up to the end, those things are already decided on and wont be changing. So we are unlikely to be wowed by anything other than the new devices. At best, there will be a few new features (like continuum) that require new hardware, but that is irrelevant to insiders as they cannot test these features nor will they be able to use them on their current device.

So far MS is ironically only listening to the bug reports, as many complaints over changes to both the interface and functions have gone either unheard or have been explained away under the excuse of "that's how it is on Android and iOS". All the complaints over performance probably are not helping either (since it drowns out actual feature suggestions and feedback on the OS. What do people expect? A fully polished beta that could pass off as RTM?).

I was hoping they would make "search" more than just a bing web wrapper, but clearly, we aren't getting more feature updates. This really sucks, because search and store are MASSIVE downgrades from WP8.1. Cortana is ugly as f*ck...

Sure would love to see or hear what amazing things C-tana is supposed to do that doesn't involve finding the best Korean/vegitarian/tapas/BBQ resturant in Mexico City. (File on my phone? I know where they are.)

Whole heartedly agree with this. The cortana system is much worse than Windows Phone 8.1, and it also looks worse than Windows 10 for PCs (ironic, no?). Even more worse on my Lumia 810, where it feels oversized.

Also aggravating that it separates smart responses from search results. Sure, on a PC one wants to be taken to their browser when doing a web search, and wants to be given the option for what kind of search they are doing (Conversation, Local device, Web search, etc.), but on a phone that's just outright unintuitive. If I search for something and Cortana has a conversational response I want to hear it, and I want the results. Now I have to choose. I also have to wait as the system identifies what I asked before I can choose (It detects whether there is a conversational query when typing and gives the option to converse with Cortana; Pretty sure voice queries default to conversation first then web search second).

Cortana is the shining example of what Windows 10 Mobile is: A PC experience "optimized" for phones. Almost every little phone friendly function and element Microsoft implemented with Windows Phone 8 has been stripped in favor of UI elements and functions that work better on PCs and tablets than they do phones for the sake of universality and being "the same" as other OSes.

Why is everyone so obsessed with this aero blur? I truly don't understand. It's on my start menu, and it's fine. I don't even think about it. What's the big difference between seeing your notifications on a black background and on a dark grey slightly blurry background?

Aesthetics. Many people care just as much about the little things that affect the aesthetics of an operating system as they are about the functionality of an operating system. I am one of those people (What? You think I chose Windows Phone for the apps?) and Im sure dubor99 here is too.

Anybody know if the Reading of Texts by Cortana both with and without Bluetooth device connection is working in this build? That's the only thing keeping me from using this as my daily driver! Here's hoping it's Yes!

After doing a full reset, my battery issues on 10166 disappeared. For each of these builds (including this new one, apparently), it's very advisable to do a full reset after upgrading so that you're not left with any remnants of a previous build's baggage. At some point, once they figure out the "upgrade" baggage problem(s), I'll install on my primary phone. Until then, I'm sticking to my trusty little Lumia 520 for testing the preview builds.

Ermmm ... shipping Windows 10 proper? There were doubtles a bunch of core lower level improvements made to Win10 upon which Mobile10 is based. Now the Mobile team have to re-stabilize their codebase atop the new foundation while also fixing lots of bugs, improve perf and security, etc.

A release such as this shortly after re-basing the code on a new substrate is not untoward. Expect some wobby stability and perf, some unusual bugs etc. Future builds should improve rapidly.

Windows RT and WP8.x were never as closely related as Win10 Mobile is to Win10. Mobile is based on 10 and the new build is even based on the TH2 branch, which is probably why they chose to not release any new features at this point - in order to stabilize on the new branch and perhaps even get more people involved in the preview.

This doesn't mean that they haven't been working on new features that just weren't included in this build.

Mobile hotspot doesn’t work in this build. When sharing the Internet from your phone using Mobile Hotspot, devices will fail to get a valid IP address from your phone and will show no internet access.

Two-factor authentication for your MSA with a phone number doesn’t work in this build. If you reset your phone and go through the first run experience, you won’t be able to input more than 2 characters for validating phone or text authentication. If you have a secondary email for two-factor authentication – you can choose that. As a workaround, add secondary email for two-factor authentication.

Some phones might spend a long time in the post update migration experience – just let the phone sit and it should complete this migration.

Apps that use background audio such as Groove, Podcast, Pandora, etc. will crash when launching after receiving an app update from the Store. Rebooting the device will resolve this issue.

Some phones with a large number of tiles pinned to the Start screen may get stuck in a state where the device shows “Loading…”; the device will need to be reset or rolled back to Windows Phone 8.1 using Windows Phone Recovery Tool.

Some apps may fail to update through the Store with the error code 0x80073cf9; uninstalling the application and reinstalling it will resolve this issue.

The Movies & TV app will not work in this build (video playback is currently broken).